every tick set the phrase number to random and then you can also do "Every X Seconds".
For the X put Random (10,20) (Or however long you want the time between attacks to be.)
and than for you action just make the boss do his attack and phrase. To make him not do the same attack twice you add another variable for "Last attack used".
Everytime the boss uses an attack set the Last Attack Used to what every phrase number that was and then add a condition to the attack checking if the new random number is NOT equal to the current one.